The history of the Breaking Bad favorite has always been shrouded in mystery, save for references to his time as a cop and his relationship with his granddaughter, Kaylee. It turns out Mike had a son, Matt, who was murdered by fellow police officers for being the only clean cop in the department. Mike retaliated by gunning down his son's killers and escaping to Albuquerque to be with his daughter-in-law (Kerry Condon) and granddaughter (Faith Healey).

The challenging episode was penned by Gordon Smith, who spent years as executive producer Vince Gilligan's writing assistant on Breaking Bad and is now a writer on Saul.

In a chat with The Hollywood Reporter, Smith revealed how he and the Saul writers crafted the Mike's backstory and teased that the ex-police officer is now on Jimmy's radar for potential criminal activity.

How much of Mike's story did you know during the Breaking Bad days? Some people assumed Kaylee's mother was Mike's daughter.

We didn't actually know it during Breaking Bad. We felt that interaction that you see in Breaking Bad felt a little too distant to be his biological daughter, and Jonathan Banks had come to us with a pitch that he had a son who was a boxer who was killed in the ring, which was interesting, and some of the DNA of that made its way into what we eventually ended up with.

There's a lot of great moments right off the bat: Mike using a maxi pad from the women's restroom, and finding that crooked vet. Where did those come from?

We wanted to give him kind of a classic, Western-style arrival. We've been watching Bad Day at Black Rock, and everybody in the room loves Once Upon a Time in the West. Those great "hero arriving on a train" moments. We were trying to come up with something that gave it some weight and some heft, so it didn't feel just like an arrival and some exposition. We knew he was coming from this shootout, and then we thought, well, maybe he was injured in this shootout. And then, how do we discover that? Maybe he's bled through his dressing. How is he going to fix that? And then it just became the fun of "all right. He walks into the women's room." Mike is the last person you would expect to go into the women's room of a public train station. So it spiraled out from that.

That shootout scene felt like Breaking Bad. What was the big challenge of getting that scene right?

That one, I knew, would be heavily under the microscope because it's such an action scene, and very much felt like a universe that we had been in for a long time. We know that whatever happens here has to be somewhat self-contained. From BreakingBad, we know that he doesn't have a prison record. He didn't get caught from doing this. So we had to make sure DNA evidence was at a minimum. We had to make sure that everything squared with what we knew about the character and what we knew about this situation. Just making sure that the details all lined up was very challenging. Does he drop the gun? Does he leave the gun? Does he wipe the gun? All that kind of stuff.

Since Mike blames himself for his son's death, we now know why he's working an honest job as a parking attendant.

Him working in the booth is almost as a penance. It's like climbing up on top of a spire in a desert for an ascetic or something like that. This is self-imposed exile out of guilt. The thing for Mike is going to be figuring out when he feels that burden has been lifted. I don't think it has been by the end of the episode. It's an open question of if he's made things right by explaining himself to his daughter-in-law. That is going to be crucial going forward in edging him out of the booth and on to where we know he's going to go in Breaking Bad.

What is his motivation for pulling that stunt to get the detective's notebook?

He knows basically what they know in terms of what evidence he's left behind. He's a guy who makes sure to dot his I's and cross his T's. The big surprise is that Stacey has called them. That's the thing that sets him off. The one thing he was looking for and try to find out is, "Is there anything in here that I don't know?" and "Is there anything in here I should be concerned about?" It's not even the evidence of the case that concerns him. It's the emotional tie that his daughter-in-law has called.

What does that interrogation scene mean for Jimmy and Mike's relationship moving forward?

It will still have the good back-and-forth sass that we've seen. But it changes Jimmy's understanding of Mike. He knew this was a capable guy, but I don't think he knew even half of what Mike was capable of. Now he's got kind of a glimpse of it. It opens up opportunities for Jimmy to consider Mike for whatever he's involved in. It opens up opportunities for him to think of him not just as a guy who happens to be useful for some information, but someone who maybe is more capable of actually performing tasks for him.

Jimmy's in his Matlock phase now, but that is going to change, and we know Mike will be a part of that.

Jimmy has discovered Mike is someone who might know things. But given the outline of what he's pretty sure Mike has done, he knows that he's somebody who can do things and get them done pretty cleanly as well.

What was your favorite scene to write?

The most fun for me to write was the final scene. It's the emotional scene, and there's so much information that has to come out. Mike is not a talker. He's a guy who uses the fewest words possible, and this is a huge monologue for him to do. Figuring out how a laconic guy is going to essentially break down and spill the beans in a way that we are not used to seeing was both a challenge and really fun.

What was it like watching Jonathan shoot it?

It was late at night. It was our last setup of the day, and he just nailed it. There was a sort of quiet when he was done. The crew applauded, and it was very emotional. He had fun with this episode and being able to play with Mike that we haven't had a chance to see yet.

What's the best advice Vince Gilligan gave you for writing your first episode?

His advice has constantly been the same, which is "don't f—k it up." (Laughs) He says it as a joke, obviously, but it's "No, don't f—k it up. Ha-ha-ha. You're going to do great. Do what you're going to do. It's there, and have confidence with it." He walks through the notes with you and is very generous with his time. He didn't really have anything other than that to say beforehand, which I think is the best vote of confidence he could have given.